1. Field of the Invention
The present invention relates to a photometric device in a camera equipped with an illumination source for generating auxiliary light in order to facilitate focus detection, and more particularly, it relates to a device for preventing the improper determination of suitable exposure conditions due to the illumination by the auxiliary light.
2. Description of the Related Art
Photometric devices are known which measure the brightness of a subject in order to perform appropriate exposure. According to the known photometric devices, a brightness value is photometrically obtained, and processing circuits in the camera determine a shutter speed and a stop value such that an appropriate exposure value for the film is determined.
The known types of photometric devices generally place emphasis in measurement on only the central portion of the image of the photometric subject. However, more recently, a method termed "divided brightness measurement" has been used to make photometric measurements wherein separate photometric measurements are made of plural portions of the photographic subject image. According to this method, an image of the photographic subject is divided into a total of five segments, including a center segment and plural peripheral segments. Photometry is performed on respective segments, and, taking into consideration the state of brightness of the principal segment and the peripheral segments, an optimum exposure condition is determined. When performing divided brightness measurement, the brightness distribution can be determined within each of the principal segment and the peripheral segments, and it becomes possible to increase the accuracy rate in determining exposure conditions. It is believed that in the future, due to improvements in light receiving elements, from tens to hundreds of divided photometric measurement regions will be possible.
Cameras having the above-described types of high performance photometric devices use built-in illumination sources in order to illuminate a subject to increase the performance of the photometric devices. For example, when a subject is dark, in order to assist in the operation of a rangefinder sensor, it is known to illuminate the subject temporarily with auxiliary light from a spotlight. Furthermore, it is also known to illuminate the subject with auxiliary light in order to prevent the "red-eye" effect.
However, in cameras equipped with a divided photometric device for divided brightness measurement and an illumination source to provide auxiliary lighting, the output of the photometric device may be affected by the auxiliary lighting. Specifically, when the divided photometric device includes a small number of segments, because each segment is relatively large in comparison with the region illuminated by the illumination source, the auxiliary light has a very small effect on the value of the photometric output. Therefore, performing exposure value calculations using the unchanged photometric output signal corresponding to each segment gives rise to only negligible error.
When the number of segments in the divided photometric device reaches tens or hundreds and the size of each segment becomes smaller, the size of the segments and the size of the region illuminated by the auxiliary light become very close. When the surface area of the photometric region corresponding to one picture element becomes narrower than the radiation region of the auxiliary light, there are effects on the photometric output value and on the exposure conditions determined from the photometric output value. For example, the problem arises that, with a true subject consisting of a point light source, a portion of the subject is illuminated solely with auxiliary light and the essentially dark subject appears bright, thereby making it impossible to distinguish the difference between the subject and the point light source, which is recognized as an error. Therefore, appropriate exposure conditions cannot be successfully obtained.